You can use it world wide, However Verizons mms settings are hardwired into the ROM so you wont be able to use mms, If you install the Access Point app you should be able to get mobile data working but no 4G, Have fun.

what is the diff between no contract lumia and unlocked lumia???online seller give two option no contract lumia and unlocked lumia??which shoud i choose why???

When phones are sold no-contract it means the phone is sold, well, without a contract.
Generally, they often are locked - but there is no service contract.

Unlocked phones are about as free as they get, free to roam from carrier to carrier - I'd pick unlocked, although with the 928, it doesn't matter.
You can enter into a contract or not with an unlocked phone, but eh.